She also failed to administer home doses of the chemotherapy and to collect prescriptions at the drugstore, police said.

"Due to Ms. LaBrie's failure to provide Jeremy with his life-saving cancer medication, his cancer has returned," police wrote.

"His cancer has returned quicker and stronger than under ordinary circumstances. He now has been diagnosed with a 10 percent chance of survival."

According to Fraser, police reports, and records in Essex Probate and Family Court, Jeremy Fraser was diagnosed with cancer in 2006 and was to be treated at Massachusetts General Hospital, where doctors told Fraser his son had a good prognosis.

The child now has fully developed leukemia, said Fraser, who was given full custody after his son's medical plight was discovered.

The couple divorced in 2005, with LaBrie receiving physical custody; Fraser had visitation rights every other weekend.

But Fraser said he was not part of his son's life between March and December 2007, a decision he said he made to protect Jeremy after his relationship with LaBrie had disintegrated after months of what he said were threats, both verbal and physical, to him and his family.

It would be better for his son, he determined, if he were to step aside.

"It just never stopped, so to continue to basically support my child, I basically had to withdraw from my son, because I knew something bad was going to happen," said Fraser, who reappeared in his son's life last Christmas. "It was a very hard decision for me to make, to walk away for that period of time."

LaBrie contended in probate court papers that Fraser willingly chose to abandon his son long before the cancer was discovered. In court papers filed in 2004, 2006, and last year, LaBrie said that Fraser was a chronic no-show for visitation.

"Father consistently does not see his child," LaBrie wrote in April 2007. "No contact with school or doctors during child's chemotherapy, no assistance with care."

LaBrie demanded an end to Fraser's parental rights, which was denied by a judge last December when she failed to show up in court, records show.

In mid-February, when LaBrie brought Jeremy to MGH for a routine appointment, doctors determined that he was not getting the care he needed and was in danger if he was allowed to return home with his mother. MGH alerted both Fraser and the Department of Social Services.

A DSS spokeswoman said yesterday that the child protection agency had been involved with the family in the past, but terminated its involvement in 2005. DSS became involved again after MGH stepped in.

Fraser said he has no idea why his former wife allegedly decided not to get her son medical care.

Dr. Robert Sege, medical director of the child protection team at Boston Medical Center, said there may be many reasons why a parent does not make sure a child gets treatment.

The parent may simply not understand how serious the consequences can be, said Sege, who played no role in Jeremy Fraser's case.

Also, he said, "people have real issues with the logistics of life - with transportation, with child care. Life doesn't stop just because you have a chronically ill kid."

Caring for an ill child is no easy job, he said.

"Families with healthy kids and working parents are stressed out as a baseline, and add to that going to multiple doctors' appointments, keeping track, being on time, making a child take a medication they may not want, and sometimes with chemo, the child is uncomfortable. All the chronic diseases of childhood are really difficult for families."

But ultimately, Sege said, "the bottom line is that the child needs to get lifesaving care."

Now that Fraser has been given full custody by the courts and LaBrie has been ordered to stay away, Fraser said he wants to spend as much time with his son as he can, given the demands of daily life.

"It comes down to quality of life," the father said.

Fraser added that he recently told LaBrie: "If you ever get a chance to see that little boy again, pick him up and tell him you're sorry."